2_Theory Related to Subsea Lif2_Theory related to subsea lifting op_Sandvik.pdfting Op_Sandvik
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Transcript of 2_Theory Related to Subsea Lif2_Theory related to subsea lifting op_Sandvik.pdfting Op_Sandvik
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MARINTEK 1
Subsea lifting operationsStavanger - Dec. 1-2 2010
Peter Chr. SandvikMARINTEK
Theory related to subsea lifting operations
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MARINTEK 2
Content Introduction, main criteria for safe lifting Lifting dynamics, simple equation of motion
Static and dynamic forces, wave forces on small objects Mass, stiffness and damping Response (motion) calculation, resonance
Large structures, 6 degrees of freedom Wave forces on long structures (e.g. pipes)
Snatch and impact loads Statistics used for estimation of extremes Stability
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MARINTEK 3
The installation often gives the highest life-time forces on subsea equipment
General
Templates, protection structures
Suction anchor
Snatch at lift-off or after slack Impact after uncontrolled
pendulous motion Local loads from wave impact
Wave forces in the splash zone
Wave forces in the splash zone Soil penetration forces (and soil
failure)
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MARINTEK 4
Lift in general
Is the structure designed for the loads occurring during lifting and deployment?
Hydrodynamic forces
Limited lifting height may give large compressive forces from the slings
Measures:
Lifting frame
Spreader beam
Reinforcement (compression bar)
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MARINTEK 5
Operation criterion:Ensure safe operation Avoid:
Excessive pendulum motion in air
Slack wire (when not intended)
Overload (in any lifting equipment)
Too hard landing (and second lift-off)
Do: Ensure acceptable stability
Have ability to handle unexpected changes
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MARINTEK 6
Static forces may be unknown Example: Drainage when H-frame is taken on board
W = mgB = gVWs = mg - gV
W = Weight in airB = BuoyancyWs = Submerged weight
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MARINTEK 7
Wreck recovery
Unknown weight,weight distributionand stability
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MARINTEK 8
Lift in air
Steel structure with GRP coversDimensions: 18 x 18 x 7 mMass: 180 tonnes
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MARINTEK 9
Wave forces in the splash zone
1
218 x 18 x 7 m, 180 tonnes
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MARINTEK 10
Wave forces in the splash zoneExample: Template (3- 8)
Large dynamic forces ( 140 T)
3 4 5
6 7 8
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MARINTEK 11
Details should be checkedExample: Template structure
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MARINTEK 12
Damaged cover
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MARINTEK 13
-15.00
-10.00
-5.00
0.00
5.00
10.00
15.00
0 500 1000 1500 2000 2500
Tension (kN)
Ver
tical
pos
ition
(m)
!
!
Wire tension when lowering a body through the splash zoneExample
}Splash zone dynamicsReducing
Weight in air
Weightin water
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MARINTEK 14
Forces on the lifted object (Newtons, or kN)
Dynamic forces:
Damping
Inertia, moving object
Inertia, wave force
Slamming force
rrrr
rd vvcvcv
vcF 210 ++=
( ) ( )xcVmxmmF aai +=+=( ) ( ) aaiw cVmVF +=+= 1
2221
ra
rss vdhdcVvAcF ==
x = body motion = wave particle motioncs = slamming coefficientca = added mass coefficient
(depth dependent)
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MARINTEK 15
Wave kinematics (1)
Profile of regular waves propagating in x direction
Wave number (deep water)
Wave length
Propagation speed
Max. wave slope
kx)t( = sin0
2
2 42Tg
k ==
256.12 Tk
==
TT
vw 56.1==
020max
4 T
kdxd
=
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MARINTEK 16
Heave force and pitch moment on a long member(It is conservative to assume small body, concentrated loads)
Wave force on an element dx:
Maximum total wave force:
dxcAf a 2)1( +=
lengthwaveLLF
F ==
sin
0
02
0 )1( acLAF +=
Harmonic wave:amplitude frequency
0
0.2
0.4
0.6
0.8
1
0 0.5 1 1.5 2 2.5 3L / Wavelength
F / F
0,
M /
F0 L
ForceMoment
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MARINTEK 17
Wave kinematics (2)Reduction with depth
Reduction of wave kinematics with depth
-100-90-80-70-60-50-40-30-20-10
0
0 0.2 0.4 0.6 0.8 1Depth reduction
Z (m
)
T = 4sT = 6sT = 10sT = 14sT = 20s
=2
4T
zkz eeR
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MARINTEK 18
Shielding-effect from the crane vesselExample of analysis results
1500
1750
2000
2250
2500
2750
3000
6 7 8 9 10 11 12Tp (s)
Max
liftw
ire fo
rce
(kN
)
Wavedir. 180 deg.Wavedir. 165 deg.Wavedir. 150 deg.
x
y21
3
45
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MARINTEK 19
Hydrodynamic data:Added mass, ma and drag , Fd 6 values - (for motion in 3 directions and rotation about 3 axis)
Plate
Box Suction anchor
Added mass coefficient:Ca = ma / V
= water densityV = reference volume
Drag coefficient: cdFd = cdAv2
v = velocityA = area
Fd
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MARINTEK 20
Added mass - simple structures
ca Vm =
b4aV
2c
=
a
b
Cylinder volume:
Geometry Formula b/a Rectan-gular
a = shortest edge
1.0 0.5791.2 0.6301.25 0.6421.33 0.6601.5 0.6912.0 0.7572.5 0.8013.0 0.8304.0 0.8715.0 0.8978.0 0.93410.0 0.947
(Cf. DnV-RP-H103)
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MARINTEK 22
Added mass of ventilated structuresExample
Added mass
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0 0.5 1 1.5 2
z(1-p)/(2Dp^2)
a/a0 Hatch 20, p=0.15
Hatch 18, p=0.25Roof #1, p=0.267Roof #2, p=0.47Roof #3, p=0.375
a = added massa0 = added mass for solid structurep = perforation ratio = open area / total areaD = dimension
Recommended values arefound in DnV-RP-H103
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MARINTEK 23
Damping
Coulomb damping Friction, hysteresis loss, ....
Linear damping Wave potential damping,
material damping, oscillation damping in wind, ....
Quadratic damping
Hydrodynamic dragMorison's formulaNotice:cd for oscillating objects is larger
than cd for steady flow !
vvcF = 00
vcF = 11
vvcF = 22
rrdd vvAcF 21=
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MARINTEK 24
Stiffness General
Axial wire stiffness E = modulus of elasticity A = section area L = length
Transverse stiffness
Hydrostatic stiffness
Rotation stiffness(spring k, distance b from
rotation center) Parallel springs
Springs in series
dxFkx
=
== ii
tot kF
k
=
==
itotitot kkkF
kF 11
LEAk =
LFFk
LFF
y
yy
yy =
==
WPAgk =
2bkK =
[Compression: k = 0]
[AWP = waterplane area]
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MARINTEK 25
Simple equation of motion
)()()()( tFxkxcxM =++
)()()()( 0 txkxkxcxM =++
Force excitation
Motion excitationF(t)
x0(t)
Force excitation Motion excitation
x
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MARINTEK 26
Response curve (RAO) of simple oscillating system
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MARINTEK 27
Resonance periods - examples
Vertical oscillation in air:
=
== 22220 gmgm
EAmLT
= elongation of wire due to weight mg
Vertical oscillation in water (long wire, wire mass mwincluded):
( )L
EAmmm
T wa 31
0 2++
=
LLgmg
mLF
mLT 22220 ===
Pendulum oscillation in air:
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MARINTEK
Example: Lifting in airAmplitude dependent transverse stiffness
28
Wire length: 25 mNatural period : 10.0 sVessel: L=100 mRegular waves H=2m
T = 5 s
T = 7 s T = 10 s
T = 9 s
T = 8 s T = 11 s
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MARINTEK
Example: Lifting in air
29
Wire length: 25 mNatural period : 10.0 sVessel: L=100 mIrregular waves Hs=2 m
Tp = 6 s
Tp = 8 s
Tp = 10 s
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MARINTEK 30
Example: Installation of suction anchorsSplash zone crossing
Two suction anchors:
Anchor B4 B30
D x H (m) 4 x 4 4 x 30
Mass, steel (t) 26 156
Added mass (t) 86 400
Total mass (t) 111 556
Submerged weight (kN) 213 1330
D
H
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MARINTEK 31
Splash zone - Anchor B30(Hs = 2.5 m, Tp = 8 s)
DAF 1.35
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MARINTEK 32
Hydrodynamic mass for suction anchorsfound from model tests
D
A = amplitudep = perforation ratioa0 = added mass,
without ventilation
Example: p = 1%A/D = 0.5
X =
X = 4950
0
0.2
0.4
0.6
0.8
1
1 10 100 1000 10000
(A/D)*(1-p)/p^2
a / a
0 p = 0.01p = 0.03p = 0.11
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MARINTEK 33
-35
-30
-25
-20
-15
-10
-5
0
0 0.2 0.4 0.6 0.8 1
Depth reduction
Z (m
)
T = 3sT = 4sT = 5sT = 6sT = 7sT = 8 sT = 9 sT = 10 sT = 11 sT = 12 sT = 14 s
Reduction of wave kinematics with depth Reduced dynamic pressure at lower end of anchor
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MARINTEK 34
Numerical modelling of a suction anchorElements with distributed forces
Horizontal added mass and damping
Vertical addedmass and damping,
topEnclosed water ++
All are positiondependent
(zero when element is above water
Anchor(steel)
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MARINTEK
Vertical wave forces on fixed buckets (Hs = 2.0 m)
35
Bucket B4 Bucket B30
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MARINTEK
Liftwire tension (Hs = 2.0 m)
36
Bucket B4 Bucket B30
DAF = 2.5 / 1.5 DAF = 1.4 / 1.3
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MARINTEK
Example: a 30x30 m stiffened foundation plate
37
M = 400 t
M = 30 t
M = 50 t
W = 3420 kNma = 15000 t
ma = 3000 t
Just fully submerged Inclination 79 deg.
Hs = 2.0 mTp = 6, 8, 10 s
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MARINTEK
Calculated forces
38
Max.: 5700 kN
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MARINTEK 39
-15.00
-10.00
-5.00
0.00
5.00
10.00
15.00
0 500 1000 1500 2000 2500
Tension (kN)
Ver
tical
pos
ition
(m)
Analysis of transient processes(Splash zone crossing, other non-linear cases)
Analysis approach
A few examples
Estimation of extreme forces
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MARINTEK 40
Lowering through the splash zoneNumerical analysis - 2 methods
Slow lowering in small, regularwaves 1 2 typical wave periods Find most onerous object position
Stationary analysis in most onerous position, in irregularwaves (0.5 - 3 hours) Calculated max./min. force
Evaluate results Study time series Find peaks in time series Statistical analysis (WEIBULL) Estimate extreme force
Repeated lowering in irregularwaves From air to well below surface Different wave realizations
(MonteCarlo random selection)
Find the calculated max./min. forcefrom each lowering case
Evaluate results Statistical analysis (GUMBEL) Study time series (explanation of
physical phenomena) Estimate extreme force
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MARINTEK 41
Statistical estimation from a time history
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MARINTEK 42
Extreme estimation from local peaks, stationary processWEIBULL plot
ln [-
ln (1
-P)]
Reduced slope due tonon-linear effects, e.g.: Quadratic damping Snatch loads
P = cumulative probabilityx = local peaks = average value of peaks
off points may be due to statistical spread,or they may have aphysical explanation.
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MARINTEK 43
Extreme estimation from repeated tests, random processExtremes from 20 lowerings through splash zone, GUMBEL plot
Cumulative probability
-1.5-1
-0.50
0.51
1.52
2.53
3.54
900 1000 1100 1200 1300 1400 1500Tension (kN)
-ln(-l
n(P)
)
off points may be due to statistical spread,OR:may represent a differentphysical mechanism(example: impact, snatch).
.95.90
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MARINTEK
Snatch loads -Example: Lift-off from transport vessel
45
Blind lift-off at max.relative velocity:
Fmax = approx. 5600 kN
Start in constant-tension modeLift when winch stops to pay in
Mass: 365 tonnesHs : 2.5 m
P = 0.95
P = 0.80
+=
+=
Mk
gVMg
MkVMgF
rel
rel
1
max
General expression:
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MARINTEK
Example: Personnel transfer to offshore wind turbines
46
Intention:The boat should hang onfender friction during personnel transfer.
All slippages were considered as events. Statistics by use of GUMBEL plots
.90.95
.80
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MARINTEK 47
Assume:Centre of buoyancy, CB, off centre of gravity, CG.
The force centre,
will be vertically below the hookheel angle
If center of vertical added mass is not inline with F:vertical excitation tilting oscillations
Lifting at points below CG should be analysed with care
Stability -Tilt angle and angular oscillations
B
CF CG CB
F = mg-B
mg
BmgCGmgCBBCF
=
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MARINTEK 48
Underwater lifting operation(Not intended)
(Stability of the lifting body)
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MARINTEK 49
Safe Job Analysis
Can anything go wrong ?
Shhh, Zog! ....Here come one now!
Subsea lifting operations Stavanger - Dec. 1-2 2010ContentThe installation often gives the highest life-time forces on subsea equipmentLift in generalOperation criterion:Ensure safe operationStatic forces may be unknown Example: Drainage when H-frame is taken on boardWreck recoveryLift in airWave forces in the splash zone Wave forces in the splash zone Example: Template (3- 8) Details should be checked Example: Template structureDamaged coverWire tension when lowering a body through the splash zoneExampleForces on the lifted object(Newtons, or kN)Wave kinematics(1)Heave force and pitch moment on a long member(It is conservative to assume small body, concentrated loads)Wave kinematics(2)Reduction with depthShielding-effect from the crane vesselExample of analysis resultsHydrodynamic data:Added mass, ma and drag , Fd 6 values - (for motion in 3 directions and rotation about 3 axis)Added mass - simple structuresAdded mass of ventilated structures ExampleDampingStiffnessSimple equation of motionResponse curve (RAO) of simple oscillating systemResonance periods - examplesExample: Lifting in airAmplitude dependent transverse stiffnessExample: Lifting in airExample: Installation of suction anchorsSplash zone crossingSplash zone - Anchor B30 (Hs = 2.5 m, Tp = 8 s)Hydrodynamic mass for suction anchorsfound from model testsReduction of wave kinematics with depth Reduced dynamic pressure at lower end of anchorNumerical modelling of a suction anchorElements with distributed forcesVertical wave forces on fixed buckets (Hs = 2.0 m)Liftwire tension (Hs = 2.0 m)Example: a 30x30 m stiffened foundation plateCalculated forcesAnalysis of transient processes (Splash zone crossing, other non-linear cases)Lowering through the splash zone Numerical analysis - 2 methodsStatistical estimation from a time historyExtreme estimation from local peaks, stationary processWEIBULL plot Extreme estimation from repeated tests, random processExtremes from 20 lowerings through splash zone, GUMBEL plotSnatch loads - Example: Lift-off from transport vesselExample: Personnel transfer to offshore wind turbinesStability - Tilt angle and angular oscillationsUnderwater lifting operation (Not intended)Safe Job AnalysisCan anything go wrong ?